Alphaviruses
Basic Information 
Epidemiology
Contagion and Zoonosis
• Although these viruses cause similar clinical syndromes in horses, the consequences of the infections they cause in humans differ.
• EEE is the most severe of the arboviral encephalitides, with case fatality rates of 50% to 70% and neurologic sequelae common in survivors.
• WEE virus appears to be less neuroinvasive but has pathology similar to EEE in patients with encephalitis.
• In adults, the VEE virus usually causes an acute, febrile, incapacitating disease with prolonged convalescence. In children, severe encephalitis may result from VEE.
Geography and Seasonality
• As with most of the alphaviruses, EEE, WEE, and VEE are transmitted by mosquitoes and maintained in cycles with various vertebrate hosts. Environmental factors that affect the interactions of the relevant mosquito and reservoir host populations control the natural epidemiology of these viruses. Some overlap of geographic extent exists.
• Outbreaks of EEE virus have occurred in most eastern states and in southeastern Canada but have been concentrated along the eastern and Gulf coasts.
• Outbreaks of WEE have been reported in the western and north-central United States, as well as Saskatchewan and Manitoba in Canada. Predominates west of the Mississippi River.
• VEE was originally reported in Venezuela, Colombia, Peru, and Ecuador before spreading to all of Central America and subsequently continuing north to Mexico and into Texas. Between active epizootics, it is not possible to isolate the equine virulent viruses with several attenuated, antigenically different VEE strains isolated instead. These enzootic strains can be differentiated among themselves and from the epizootic strains.
Clinical Presentation
Physical Exam Findings
• Clinical signs not pathognomonic
• Fever associated with viremia in majority of infected horses
• Initial hyperexcitability, progressing to depression and recumbency
• Compulsive walking, circling
• Permanent neurologic deficits may occur in survivors
• Early neurologic signs reflect diffuse, multifocal cortical disease
• Mortality ranges from 75% to 90% for EEE, 19% to 50% for WEE, and 40% to 90% for VEE
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